This invention relates to the preparation of active dried yeasts, and is more particularly concerned with active dried yeasts, having improved leavening activity and with processes for their preparation.
Active dried yeasts are prepared by fermentation of the appropriate strain of yeast in the presence of necessary nutrients, followed by isolation of the yeast cells from the fermentation broth to obtain a yeast cream, dewatering the yeast cream by filtration, and drying of the resulting yeast filter cake under controlled conditions.
Dewatering the yeast cream prior to drying is typically carried out by either a filter press or vacuum filtration. When using a vacuum filter to dewater the yeast cream, an important step involves treating the yeast cream with an aqueous solution of a salt such as sodium chloride, potassium chloride, or the like, which by osmotic action, forces water from the cells of the yeast. After such treatment, the majority of the external aqueous solution is separated from the wet yeast by filtration, during which process the yeast is washed with water to remove excess salt.
The compressed yeast is then subjected to drying by any of a variety of techniques such as those described in U.S. Pat. No. 3,617,306 which exemplifies the so-called "spaghetti" method. Alternative procedures for drying of yeast are described, for example, in U.S. Pat. Nos. 3,843,800 and 4,217,420.
A wide variety of osmotic solutions have been employed previously in order to force water from the cells of yeast by osmotic action. Illustratively, U.S. Pat. No. 2,947,668 teaches the use of water soluble salts of sodium, potassium, ammonium, calcium, magnesium and aluminum as well as non-electrolytes such as water soluble mono and polyhydric alcohols. The use of these various osmotic agents was contemplated by the above-identified reference in the preparation of compressed yeast rather than active dried yeast. The two forms of yeast are prepared from different types of cultures. The type of culture generally utilized in the preparation of compressed yeast products is classified as Bios No. 236 while that generally utilized in the preparation of active dried yeast is classified as Bios No. 23. These classifications are made according to the bios response procedure published by Schultz and Atkin in "Archives of Biochemistry" Vol. 14, p. 369, Aug., 1947.
The osmotic agent is selected typically on the basis of cost and osmotic potential. Those most commonly employed in the art are potassium and sodium chlorides. It is well recognized in the art that the production of active dried yeast by drying of yeast cake in the above-identified manner is accompanied by significant loss of activity (as determined by standard tests). However, it has not been previously recognized that the choice of the particular salts employed in the initial treatment of the yeast cream can play a part in controlling the loss of activity in the drying process to produce active dried yeast.
We have now found that the use of certain divalent metal salts in the solutions used to remove water from the yeast cells by osmotic action can substantially reduce, and in many cases, eliminate the loss in activity encountered hitherto in the drying process required to produce active dried yeast.